1Title : Kernel Probes (Kprobes)
2Authors : Jim Keniston <jkenisto@us.ibm.com>
3 : Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
4 : Masami Hiramatsu <mhiramat@redhat.com>
5 6CONTENTS
7 81. Concepts: Kprobes, Jprobes, Return Probes
92. Architectures Supported
103. Configuring Kprobes
114. API Reference
125. Kprobes Features and Limitations
136. Probe Overhead
147. TODO
158. Kprobes Example
169. Jprobes Example
1710. Kretprobes Example
18Appendix A: The kprobes debugfs interface
19Appendix B: The kprobes sysctl interface
20 211. Concepts: Kprobes, Jprobes, Return Probes
22 23Kprobes enables you to dynamically break into any kernel routine and
24collect debugging and performance information non-disruptively. You
25can trap at almost any kernel code address(*), specifying a handler
26routine to be invoked when the breakpoint is hit.
27(*: some parts of the kernel code can not be trapped, see 1.5 Blacklist)
28 29There are currently three types of probes: kprobes, jprobes, and
30kretprobes (also called return probes). A kprobe can be inserted
31on virtually any instruction in the kernel. A jprobe is inserted at
32the entry to a kernel function, and provides convenient access to the
33function's arguments. A return probe fires when a specified function
34returns.
35 36In the typical case, Kprobes-based instrumentation is packaged as
37a kernel module. The module's init function installs ("registers")
38one or more probes, and the exit function unregisters them. A
39registration function such as register_kprobe() specifies where
40the probe is to be inserted and what handler is to be called when
41the probe is hit.
42 43There are also register_/unregister_*probes() functions for batch
44registration/unregistration of a group of *probes. These functions
45can speed up unregistration process when you have to unregister
46a lot of probes at once.
47 48The next four subsections explain how the different types of
49probes work and how jump optimization works. They explain certain
50things that you'll need to know in order to make the best use of
51Kprobes -- e.g., the difference between a pre_handler and
52a post_handler, and how to use the maxactive and nmissed fields of
53a kretprobe. But if you're in a hurry to start using Kprobes, you
54can skip ahead to section 2.
55 561.1 How Does a Kprobe Work?
57 58When a kprobe is registered, Kprobes makes a copy of the probed
59instruction and replaces the first byte(s) of the probed instruction
60with a breakpoint instruction (e.g., int3 on i386 and x86_64).
61 62When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
63registers are saved, and control passes to Kprobes via the
64notifier_call_chain mechanism. Kprobes executes the "pre_handler"
65associated with the kprobe, passing the handler the addresses of the
66kprobe struct and the saved registers.
67 68Next, Kprobes single-steps its copy of the probed instruction.
69(It would be simpler to single-step the actual instruction in place,
70but then Kprobes would have to temporarily remove the breakpoint
71instruction. This would open a small time window when another CPU
72could sail right past the probepoint.)
73 74After the instruction is single-stepped, Kprobes executes the
75"post_handler," if any, that is associated with the kprobe.
76Execution then continues with the instruction following the probepoint.
77 781.2 How Does a Jprobe Work?
79 80A jprobe is implemented using a kprobe that is placed on a function's
81entry point. It employs a simple mirroring principle to allow
82seamless access to the probed function's arguments. The jprobe
83handler routine should have the same signature (arg list and return
84type) as the function being probed, and must always end by calling
85the Kprobes function jprobe_return().
86 87Here's how it works. When the probe is hit, Kprobes makes a copy of
88the saved registers and a generous portion of the stack (see below).
89Kprobes then points the saved instruction pointer at the jprobe's
90handler routine, and returns from the trap. As a result, control
91passes to the handler, which is presented with the same register and
92stack contents as the probed function. When it is done, the handler
93calls jprobe_return(), which traps again to restore the original stack
94contents and processor state and switch to the probed function.
95 96By convention, the callee owns its arguments, so gcc may produce code
97that unexpectedly modifies that portion of the stack. This is why
98Kprobes saves a copy of the stack and restores it after the jprobe
99handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
10064 bytes on i386.
101 102Note that the probed function's args may be passed on the stack
103or in registers. The jprobe will work in either case, so long as the
104handler's prototype matches that of the probed function.
105 106Note that in some architectures (e.g.: arm64 and sparc64) the stack
107copy is not done, as the actual location of stacked parameters may be
108outside of a reasonable MAX_STACK_SIZE value and because that location
109cannot be determined by the jprobes code. In this case the jprobes
110user must be careful to make certain the calling signature of the
111function does not cause parameters to be passed on the stack (e.g.:
112more than eight function arguments, an argument of more than sixteen
113bytes, or more than 64 bytes of argument data, depending on
114architecture).
115 1161.3 Return Probes
117 1181.3.1 How Does a Return Probe Work?
119 120When you call register_kretprobe(), Kprobes establishes a kprobe at
121the entry to the function. When the probed function is called and this
122probe is hit, Kprobes saves a copy of the return address, and replaces
123the return address with the address of a "trampoline." The trampoline
124is an arbitrary piece of code -- typically just a nop instruction.
125At boot time, Kprobes registers a kprobe at the trampoline.
126 127When the probed function executes its return instruction, control
128passes to the trampoline and that probe is hit. Kprobes' trampoline
129handler calls the user-specified return handler associated with the
130kretprobe, then sets the saved instruction pointer to the saved return
131address, and that's where execution resumes upon return from the trap.
132 133While the probed function is executing, its return address is
134stored in an object of type kretprobe_instance. Before calling
135register_kretprobe(), the user sets the maxactive field of the
136kretprobe struct to specify how many instances of the specified
137function can be probed simultaneously. register_kretprobe()
138pre-allocates the indicated number of kretprobe_instance objects.
139 140For example, if the function is non-recursive and is called with a
141spinlock held, maxactive = 1 should be enough. If the function is
142non-recursive and can never relinquish the CPU (e.g., via a semaphore
143or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
144set to a default value. If CONFIG_PREEMPT is enabled, the default
145is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
146 147It's not a disaster if you set maxactive too low; you'll just miss
148some probes. In the kretprobe struct, the nmissed field is set to
149zero when the return probe is registered, and is incremented every
150time the probed function is entered but there is no kretprobe_instance
151object available for establishing the return probe.
152 1531.3.2 Kretprobe entry-handler
154 155Kretprobes also provides an optional user-specified handler which runs
156on function entry. This handler is specified by setting the entry_handler
157field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
158function entry is hit, the user-defined entry_handler, if any, is invoked.
159If the entry_handler returns 0 (success) then a corresponding return handler
160is guaranteed to be called upon function return. If the entry_handler
161returns a non-zero error then Kprobes leaves the return address as is, and
162the kretprobe has no further effect for that particular function instance.
163 164Multiple entry and return handler invocations are matched using the unique
165kretprobe_instance object associated with them. Additionally, a user
166may also specify per return-instance private data to be part of each
167kretprobe_instance object. This is especially useful when sharing private
168data between corresponding user entry and return handlers. The size of each
169private data object can be specified at kretprobe registration time by
170setting the data_size field of the kretprobe struct. This data can be
171accessed through the data field of each kretprobe_instance object.
172 173In case probed function is entered but there is no kretprobe_instance
174object available, then in addition to incrementing the nmissed count,
175the user entry_handler invocation is also skipped.
176 1771.4 How Does Jump Optimization Work?
178 179If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
180is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
181the "debug.kprobes_optimization" kernel parameter is set to 1 (see
182sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
183instruction instead of a breakpoint instruction at each probepoint.
184 1851.4.1 Init a Kprobe
186 187When a probe is registered, before attempting this optimization,
188Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
189address. So, even if it's not possible to optimize this particular
190probepoint, there'll be a probe there.
191 1921.4.2 Safety Check
193 194Before optimizing a probe, Kprobes performs the following safety checks:
195 196- Kprobes verifies that the region that will be replaced by the jump
197instruction (the "optimized region") lies entirely within one function.
198(A jump instruction is multiple bytes, and so may overlay multiple
199instructions.)
200 201- Kprobes analyzes the entire function and verifies that there is no
202jump into the optimized region. Specifically:
203 - the function contains no indirect jump;
204 - the function contains no instruction that causes an exception (since
205 the fixup code triggered by the exception could jump back into the
206 optimized region -- Kprobes checks the exception tables to verify this);
207 and
208 - there is no near jump to the optimized region (other than to the first
209 byte).
210 211- For each instruction in the optimized region, Kprobes verifies that
212the instruction can be executed out of line.
213 2141.4.3 Preparing Detour Buffer
215 216Next, Kprobes prepares a "detour" buffer, which contains the following
217instruction sequence:
218- code to push the CPU's registers (emulating a breakpoint trap)
219- a call to the trampoline code which calls user's probe handlers.
220- code to restore registers
221- the instructions from the optimized region
222- a jump back to the original execution path.
223 2241.4.4 Pre-optimization
225 226After preparing the detour buffer, Kprobes verifies that none of the
227following situations exist:
228- The probe has either a break_handler (i.e., it's a jprobe) or a
229post_handler.
230- Other instructions in the optimized region are probed.
231- The probe is disabled.
232In any of the above cases, Kprobes won't start optimizing the probe.
233Since these are temporary situations, Kprobes tries to start
234optimizing it again if the situation is changed.
235 236If the kprobe can be optimized, Kprobes enqueues the kprobe to an
237optimizing list, and kicks the kprobe-optimizer workqueue to optimize
238it. If the to-be-optimized probepoint is hit before being optimized,
239Kprobes returns control to the original instruction path by setting
240the CPU's instruction pointer to the copied code in the detour buffer
241-- thus at least avoiding the single-step.
242 2431.4.5 Optimization
244 245The Kprobe-optimizer doesn't insert the jump instruction immediately;
246rather, it calls synchronize_sched() for safety first, because it's
247possible for a CPU to be interrupted in the middle of executing the
248optimized region(*). As you know, synchronize_sched() can ensure
249that all interruptions that were active when synchronize_sched()
250was called are done, but only if CONFIG_PREEMPT=n. So, this version
251of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
252 253After that, the Kprobe-optimizer calls stop_machine() to replace
254the optimized region with a jump instruction to the detour buffer,
255using text_poke_smp().
256 2571.4.6 Unoptimization
258 259When an optimized kprobe is unregistered, disabled, or blocked by
260another kprobe, it will be unoptimized. If this happens before
261the optimization is complete, the kprobe is just dequeued from the
262optimized list. If the optimization has been done, the jump is
263replaced with the original code (except for an int3 breakpoint in
264the first byte) by using text_poke_smp().
265 266(*)Please imagine that the 2nd instruction is interrupted and then
267the optimizer replaces the 2nd instruction with the jump *address*
268while the interrupt handler is running. When the interrupt
269returns to original address, there is no valid instruction,
270and it causes an unexpected result.
271 272(**)This optimization-safety checking may be replaced with the
273stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
274kernel.
275 276NOTE for geeks:
277The jump optimization changes the kprobe's pre_handler behavior.
278Without optimization, the pre_handler can change the kernel's execution
279path by changing regs->ip and returning 1. However, when the probe
280is optimized, that modification is ignored. Thus, if you want to
281tweak the kernel's execution path, you need to suppress optimization,
282using one of the following techniques:
283- Specify an empty function for the kprobe's post_handler or break_handler.
284 or
285- Execute 'sysctl -w debug.kprobes_optimization=n'
286 2871.5 Blacklist
288 289Kprobes can probe most of the kernel except itself. This means
290that there are some functions where kprobes cannot probe. Probing
291(trapping) such functions can cause a recursive trap (e.g. double
292fault) or the nested probe handler may never be called.
293Kprobes manages such functions as a blacklist.
294If you want to add a function into the blacklist, you just need
295to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
296to specify a blacklisted function.
297Kprobes checks the given probe address against the blacklist and
298rejects registering it, if the given address is in the blacklist.
299 3002. Architectures Supported
301 302Kprobes, jprobes, and return probes are implemented on the following
303architectures:
304 305- i386 (Supports jump optimization)
306- x86_64 (AMD-64, EM64T) (Supports jump optimization)
307- ppc64
308- ia64 (Does not support probes on instruction slot1.)
309- sparc64 (Return probes not yet implemented.)
310- arm
311- ppc
312- mips
313- s390
314 3153. Configuring Kprobes
316 317When configuring the kernel using make menuconfig/xconfig/oldconfig,
318ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
319for "Kprobes".
320 321So that you can load and unload Kprobes-based instrumentation modules,
322make sure "Loadable module support" (CONFIG_MODULES) and "Module
323unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
324 325Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
326are set to "y", since kallsyms_lookup_name() is used by the in-kernel
327kprobe address resolution code.
328 329If you need to insert a probe in the middle of a function, you may find
330it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
331so you can use "objdump -d -l vmlinux" to see the source-to-object
332code mapping.
333 3344. API Reference
335 336The Kprobes API includes a "register" function and an "unregister"
337function for each type of probe. The API also includes "register_*probes"
338and "unregister_*probes" functions for (un)registering arrays of probes.
339Here are terse, mini-man-page specifications for these functions and
340the associated probe handlers that you'll write. See the files in the
341samples/kprobes/ sub-directory for examples.
342 3434.1 register_kprobe
344 345#include <linux/kprobes.h>
346int register_kprobe(struct kprobe *kp);
347 348Sets a breakpoint at the address kp->addr. When the breakpoint is
349hit, Kprobes calls kp->pre_handler. After the probed instruction
350is single-stepped, Kprobe calls kp->post_handler. If a fault
351occurs during execution of kp->pre_handler or kp->post_handler,
352or during single-stepping of the probed instruction, Kprobes calls
353kp->fault_handler. Any or all handlers can be NULL. If kp->flags
354is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
355so, its handlers aren't hit until calling enable_kprobe(kp).
356 357NOTE:
3581. With the introduction of the "symbol_name" field to struct kprobe,
359the probepoint address resolution will now be taken care of by the kernel.
360The following will now work:
361 362 kp.symbol_name = "symbol_name";
363 364(64-bit powerpc intricacies such as function descriptors are handled
365transparently)
366 3672. Use the "offset" field of struct kprobe if the offset into the symbol
368to install a probepoint is known. This field is used to calculate the
369probepoint.
370 3713. Specify either the kprobe "symbol_name" OR the "addr". If both are
372specified, kprobe registration will fail with -EINVAL.
373 3744. With CISC architectures (such as i386 and x86_64), the kprobes code
375does not validate if the kprobe.addr is at an instruction boundary.
376Use "offset" with caution.
377 378register_kprobe() returns 0 on success, or a negative errno otherwise.
379 380User's pre-handler (kp->pre_handler):
381#include <linux/kprobes.h>
382#include <linux/ptrace.h>
383int pre_handler(struct kprobe *p, struct pt_regs *regs);
384 385Called with p pointing to the kprobe associated with the breakpoint,
386and regs pointing to the struct containing the registers saved when
387the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
388 389User's post-handler (kp->post_handler):
390#include <linux/kprobes.h>
391#include <linux/ptrace.h>
392void post_handler(struct kprobe *p, struct pt_regs *regs,
393 unsigned long flags);
394 395p and regs are as described for the pre_handler. flags always seems
396to be zero.
397 398User's fault-handler (kp->fault_handler):
399#include <linux/kprobes.h>
400#include <linux/ptrace.h>
401int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
402 403p and regs are as described for the pre_handler. trapnr is the
404architecture-specific trap number associated with the fault (e.g.,
405on i386, 13 for a general protection fault or 14 for a page fault).
406Returns 1 if it successfully handled the exception.
407 4084.2 register_jprobe
409 410#include <linux/kprobes.h>
411int register_jprobe(struct jprobe *jp)
412 413Sets a breakpoint at the address jp->kp.addr, which must be the address
414of the first instruction of a function. When the breakpoint is hit,
415Kprobes runs the handler whose address is jp->entry.
416 417The handler should have the same arg list and return type as the probed
418function; and just before it returns, it must call jprobe_return().
419(The handler never actually returns, since jprobe_return() returns
420control to Kprobes.) If the probed function is declared asmlinkage
421or anything else that affects how args are passed, the handler's
422declaration must match.
423 424register_jprobe() returns 0 on success, or a negative errno otherwise.
425 4264.3 register_kretprobe
427 428#include <linux/kprobes.h>
429int register_kretprobe(struct kretprobe *rp);
430 431Establishes a return probe for the function whose address is
432rp->kp.addr. When that function returns, Kprobes calls rp->handler.
433You must set rp->maxactive appropriately before you call
434register_kretprobe(); see "How Does a Return Probe Work?" for details.
435 436register_kretprobe() returns 0 on success, or a negative errno
437otherwise.
438 439User's return-probe handler (rp->handler):
440#include <linux/kprobes.h>
441#include <linux/ptrace.h>
442int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
443 444regs is as described for kprobe.pre_handler. ri points to the
445kretprobe_instance object, of which the following fields may be
446of interest:
447- ret_addr: the return address
448- rp: points to the corresponding kretprobe object
449- task: points to the corresponding task struct
450- data: points to per return-instance private data; see "Kretprobe
451 entry-handler" for details.
452 453The regs_return_value(regs) macro provides a simple abstraction to
454extract the return value from the appropriate register as defined by
455the architecture's ABI.
456 457The handler's return value is currently ignored.
458 4594.4 unregister_*probe
460 461#include <linux/kprobes.h>
462void unregister_kprobe(struct kprobe *kp);
463void unregister_jprobe(struct jprobe *jp);
464void unregister_kretprobe(struct kretprobe *rp);
465 466Removes the specified probe. The unregister function can be called
467at any time after the probe has been registered.
468 469NOTE:
470If the functions find an incorrect probe (ex. an unregistered probe),
471they clear the addr field of the probe.
472 4734.5 register_*probes
474 475#include <linux/kprobes.h>
476int register_kprobes(struct kprobe **kps, int num);
477int register_kretprobes(struct kretprobe **rps, int num);
478int register_jprobes(struct jprobe **jps, int num);
479 480Registers each of the num probes in the specified array. If any
481error occurs during registration, all probes in the array, up to
482the bad probe, are safely unregistered before the register_*probes
483function returns.
484- kps/rps/jps: an array of pointers to *probe data structures
485- num: the number of the array entries.
486 487NOTE:
488You have to allocate(or define) an array of pointers and set all
489of the array entries before using these functions.
490 4914.6 unregister_*probes
492 493#include <linux/kprobes.h>
494void unregister_kprobes(struct kprobe **kps, int num);
495void unregister_kretprobes(struct kretprobe **rps, int num);
496void unregister_jprobes(struct jprobe **jps, int num);
497 498Removes each of the num probes in the specified array at once.
499 500NOTE:
501If the functions find some incorrect probes (ex. unregistered
502probes) in the specified array, they clear the addr field of those
503incorrect probes. However, other probes in the array are
504unregistered correctly.
505 5064.7 disable_*probe
507 508#include <linux/kprobes.h>
509int disable_kprobe(struct kprobe *kp);
510int disable_kretprobe(struct kretprobe *rp);
511int disable_jprobe(struct jprobe *jp);
512 513Temporarily disables the specified *probe. You can enable it again by using
514enable_*probe(). You must specify the probe which has been registered.
515 5164.8 enable_*probe
517 518#include <linux/kprobes.h>
519int enable_kprobe(struct kprobe *kp);
520int enable_kretprobe(struct kretprobe *rp);
521int enable_jprobe(struct jprobe *jp);
522 523Enables *probe which has been disabled by disable_*probe(). You must specify
524the probe which has been registered.
525 5265. Kprobes Features and Limitations
527 528Kprobes allows multiple probes at the same address. Currently,
529however, there cannot be multiple jprobes on the same function at
530the same time. Also, a probepoint for which there is a jprobe or
531a post_handler cannot be optimized. So if you install a jprobe,
532or a kprobe with a post_handler, at an optimized probepoint, the
533probepoint will be unoptimized automatically.
534 535In general, you can install a probe anywhere in the kernel.
536In particular, you can probe interrupt handlers. Known exceptions
537are discussed in this section.
538 539The register_*probe functions will return -EINVAL if you attempt
540to install a probe in the code that implements Kprobes (mostly
541kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
542as do_page_fault and notifier_call_chain).
543 544If you install a probe in an inline-able function, Kprobes makes
545no attempt to chase down all inline instances of the function and
546install probes there. gcc may inline a function without being asked,
547so keep this in mind if you're not seeing the probe hits you expect.
548 549A probe handler can modify the environment of the probed function
550-- e.g., by modifying kernel data structures, or by modifying the
551contents of the pt_regs struct (which are restored to the registers
552upon return from the breakpoint). So Kprobes can be used, for example,
553to install a bug fix or to inject faults for testing. Kprobes, of
554course, has no way to distinguish the deliberately injected faults
555from the accidental ones. Don't drink and probe.
556 557Kprobes makes no attempt to prevent probe handlers from stepping on
558each other -- e.g., probing printk() and then calling printk() from a
559probe handler. If a probe handler hits a probe, that second probe's
560handlers won't be run in that instance, and the kprobe.nmissed member
561of the second probe will be incremented.
562 563As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
564the same handler) may run concurrently on different CPUs.
565 566Kprobes does not use mutexes or allocate memory except during
567registration and unregistration.
568 569Probe handlers are run with preemption disabled. Depending on the
570architecture and optimization state, handlers may also run with
571interrupts disabled (e.g., kretprobe handlers and optimized kprobe
572handlers run without interrupt disabled on x86/x86-64). In any case,
573your handler should not yield the CPU (e.g., by attempting to acquire
574a semaphore).
575 576Since a return probe is implemented by replacing the return
577address with the trampoline's address, stack backtraces and calls
578to __builtin_return_address() will typically yield the trampoline's
579address instead of the real return address for kretprobed functions.
580(As far as we can tell, __builtin_return_address() is used only
581for instrumentation and error reporting.)
582 583If the number of times a function is called does not match the number
584of times it returns, registering a return probe on that function may
585produce undesirable results. In such a case, a line:
586kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
587gets printed. With this information, one will be able to correlate the
588exact instance of the kretprobe that caused the problem. We have the
589do_exit() case covered. do_execve() and do_fork() are not an issue.
590We're unaware of other specific cases where this could be a problem.
591 592If, upon entry to or exit from a function, the CPU is running on
593a stack other than that of the current task, registering a return
594probe on that function may produce undesirable results. For this
595reason, Kprobes doesn't support return probes (or kprobes or jprobes)
596on the x86_64 version of __switch_to(); the registration functions
597return -EINVAL.
598 599On x86/x86-64, since the Jump Optimization of Kprobes modifies
600instructions widely, there are some limitations to optimization. To
601explain it, we introduce some terminology. Imagine a 3-instruction
602sequence consisting of a two 2-byte instructions and one 3-byte
603instruction.
604 605 IA
606 |
607[-2][-1][0][1][2][3][4][5][6][7]
608 [ins1][ins2][ ins3 ]
609 [<- DCR ->]
610 [<- JTPR ->]
611 612ins1: 1st Instruction
613ins2: 2nd Instruction
614ins3: 3rd Instruction
615IA: Insertion Address
616JTPR: Jump Target Prohibition Region
617DCR: Detoured Code Region
618 619The instructions in DCR are copied to the out-of-line buffer
620of the kprobe, because the bytes in DCR are replaced by
621a 5-byte jump instruction. So there are several limitations.
622 623a) The instructions in DCR must be relocatable.
624b) The instructions in DCR must not include a call instruction.
625c) JTPR must not be targeted by any jump or call instruction.
626d) DCR must not straddle the border between functions.
627 628Anyway, these limitations are checked by the in-kernel instruction
629decoder, so you don't need to worry about that.
630 6316. Probe Overhead
632 633On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
634microseconds to process. Specifically, a benchmark that hits the same
635probepoint repeatedly, firing a simple handler each time, reports 1-2
636million hits per second, depending on the architecture. A jprobe or
637return-probe hit typically takes 50-75% longer than a kprobe hit.
638When you have a return probe set on a function, adding a kprobe at
639the entry to that function adds essentially no overhead.
640 641Here are sample overhead figures (in usec) for different architectures.
642k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
643on same function; jr = jprobe + return probe on same function
644 645i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
646k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
647 648x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
649k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
650 651ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
652k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
653 6546.1 Optimized Probe Overhead
655 656Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
657process. Here are sample overhead figures (in usec) for x86 architectures.
658k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
659r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
660 661i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
662k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
663 664x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
665k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
666 6677. TODO
668 669a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
670programming interface for probe-based instrumentation. Try it out.
671b. Kernel return probes for sparc64.
672c. Support for other architectures.
673d. User-space probes.
674e. Watchpoint probes (which fire on data references).
675 6768. Kprobes Example
677 678See samples/kprobes/kprobe_example.c
679 6809. Jprobes Example
681 682See samples/kprobes/jprobe_example.c
683 68410. Kretprobes Example
685 686See samples/kprobes/kretprobe_example.c
687 688For additional information on Kprobes, refer to the following URLs:
689http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe 690http://www.redhat.com/magazine/005mar05/features/kprobes/ 691http://www-users.cs.umn.edu/~boutcher/kprobes/ 692http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
693 694 695Appendix A: The kprobes debugfs interface
696 697With recent kernels (> 2.6.20) the list of registered kprobes is visible
698under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
699 700/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
701 702c015d71a k vfs_read+0x0
703c011a316 j do_fork+0x0
704c03dedc5 r tcp_v4_rcv+0x0
705 706The first column provides the kernel address where the probe is inserted.
707The second column identifies the type of probe (k - kprobe, r - kretprobe
708and j - jprobe), while the third column specifies the symbol+offset of
709the probe. If the probed function belongs to a module, the module name
710is also specified. Following columns show probe status. If the probe is on
711a virtual address that is no longer valid (module init sections, module
712virtual addresses that correspond to modules that've been unloaded),
713such probes are marked with [GONE]. If the probe is temporarily disabled,
714such probes are marked with [DISABLED]. If the probe is optimized, it is
715marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
716[FTRACE].
717 718/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
719 720Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
721By default, all kprobes are enabled. By echoing "0" to this file, all
722registered probes will be disarmed, till such time a "1" is echoed to this
723file. Note that this knob just disarms and arms all kprobes and doesn't
724change each probe's disabling state. This means that disabled kprobes (marked
725[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
726 727 728Appendix B: The kprobes sysctl interface
729 730/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
731 732When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
733a knob to globally and forcibly turn jump optimization (see section
7341.4) ON or OFF. By default, jump optimization is allowed (ON).
735If you echo "0" to this file or set "debug.kprobes_optimization" to
7360 via sysctl, all optimized probes will be unoptimized, and any new
737probes registered after that will not be optimized. Note that this
738knob *changes* the optimized state. This means that optimized probes
739(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
740removed). If the knob is turned on, they will be optimized again.
741 742